Research On The Control Strategy Of String Type PV Grid-connected Inverter

Centralized photovoltaic grid-connected power generation system has slowed down its development speed in recent years because of its system structure and high maintenance cost.On the contrary,the string type photovoltaic grid-connected power generation system is the new favorite of the times and has developed rapidly in recent years.The control strategy of PV grid-connected inverter is the control core of PV grid-connected generation system.The control strategy is to realize grid-connected current quality improvement,PV power system's active and reactive power control and low voltage ride through(LVRT)capability.In order to realize the effective control of string type grid-connected inverter output current,this dissertation adopts the idea of vector transformation,transforming the three-phase static coordinate to the two-phase rotating dq coordinate system to realize active and reactive current decoupling control,and then realizing the regulation of the active and reactive power output of the photovoltaic system.In this dissertation,grid-connected current control strategy based on dq coordinate system is designed under the condition of three-phase voltage balance and three-phase unbalance,respectively.The advantage of the control strategy is that the control system is simple in structure and in principle and mature in the control system stability.When the asymmetrical fault occurs in the grid,the positive and negative sequence separation algorithm based on the cascade delay signal elimination method is used to realize the positive and negative sequence achieving separation of the active and reactive currents quickly and accurately.The positive sequence component of the fundamental current And the reference value of the grid-connected current,so that the grid current phase and frequency to maintain normal,to achieve grid-connected photovoltaic power generation systems and network synchronization.With the increase of grid-connected capacity of PV power system,PV power system needs to support the voltage recovery after grid fault elimination in the event of power grid fault,that is,PV power system needs high / low voltage traversing capability.In this dissertation,the voltage drop in different depths and different forms is simulated by impedance partial pressure method.The low voltage ride-through control strategy based on MPPT mode and Non-MPPT mode is studied.Non-MPPT mode is adopted in low voltage traversing process,-MPPT mode.The basic idea is: when the grid voltage drops,first,to determine the form of grid voltage drop,the balance drop or unbalance fall;the second step,the calculation of grid voltage or power grid fundamental sequence voltage drop depth;the third step,according to the power grid Voltage drop depth calculation reactive current command or the fundamental positive sequence reactive current command;Finally,throughthe inverter modulation and network current reactive component,in order to achieve low voltage through.In this dissertation,a LVRT control strategy for grid-connected power generation system is designed under symmetrical fault and asymmetrical fault condition.The reactive power compensation quantity of the system is calculated by the voltage drop depth of the grid,and the control strategy of the grid-connected inverter is adopted to realize the low voltage traverse of the system.In this dissertation,a simulation model of string type PV grid-connected inverter is built on MATLAB/Simulink platform,and the correctness and effectiveness of the proposed current control strategy and low voltage ride-through control strategy are verified.Finally,the simulation data and results basically reach the expected requirements.